The present invention refers to flow meters, suitable for measuring, monitoring and controlling of volume or mass flow rate of various gaseous or liquid fluids or streams of particulate materials.
More particularly the present flow meter refers to so-called magnetic flow meters, employing a floating member, made of magnetically responsive material and residing within a tubular conduit, through which the fluid flows. Such flow meters are provided with a means for producing magnetic field in the conduit and they generate a response signal upon displacement of the float member within the magnetic field by the flowing fluid. Flow rate value is obtained by processing the response signal and comparing it with the a calibration curve.
There are known various flow meters, which have been devised for measuring of flow rate and which are based on various physical principles.
For example so-called Coriolis flow meter employ a sensor tube, through which the fluid flows and twists the tube. This twisting characteristic is called the Coriolis effect. According to Newton""s Second Law of Motion, the amount of sensor tube twist is directly proportional to the mass flow rate of the fluid flowing through the tube.
The Coriolis flow meters are reliable instruments, which capable to measure wide range of flow rate of fluids from several kg to several hundreds of tons per hours with accuracy xc2x1(0.15-0.25)% of ratexc2x1zero stability. Unfortunately, their construction is rather complicated and they are expensive items.
There are known also magnetic flow meters, employing a responsive signal generated by a ferromagnetic floating member, residing in a conduit in which a magnetic field is created and is displaced by the fluid flowing through the conduit.
In DE3341661 is disclosed float-type flow meter consisting of a cylindrical non-ferromagnetic measuring tube having a baffle arranged in the interior of the tube. A conical ferromagnetic float is located in the tube and there is provided two coils arranged axially in the tube. The coils are energized by a source of alternating voltage. The coils are electrically connected to provide a differential transformer, which is capable to generate a responsive signal upon variation of inductivity in the coils when the float displaces. This signal is processed and after comparing with a pre-established calibration curve, the flow rate is derived.
In this flow meter the flow rate value depends on the relationship between the cross section of the float and the inner diameter of the measuring tube. Due to this provision the flow meter is capable to measure only small and extremely small flow rates.
Furthermore, operating of this instrument is not convenient, since it requires pre-establishing of dedicated calibration curve for each fluid, to be measured and therefore it is not possible to work with a single, universal calibration curve.
There are known also magnetic flow meters, in which magnetic force is employed to resist the drag force of the flowing fluid imposed on the float to retain the float in neutral position.
In U.S. Pat. No. 4,041,756 is described a flow meter including a vertically positioned flow tube having ferromagnetic ball residing therein. The ball is subjected to the force of gravity as well of the drag force imposed by the flow of fluid. Associated with the flow tube is a position sensor, employing a light source projecting a beam across the translucent tube and a photo sensor, yielding a control signal once the ball obscures the beam. A magnetic means is provided for producing electromagnetic field in the flow tube. The magnetic means is formed as electromagnet consisting of electrically conductive wire coil wound around a core. The electromagnet is energized by a current controller and governed by the control signal. The electromagnet produces a magnetic force, seeking to displace the ball and retain thereof away from the light beam. Thus the ball is caused to reside the position depending on the vector resultant of the co-acting drag force, produced by the fluid, gravitational and magnetic force. The time average current generated by the controller is a function of the flow rate.
This flow meter has number of intrinsic disadvantages. Since it employs optical sensor it requires that the measuring tube be made of transparent material and therefore the operation is limited strictly to measurement of flow rate of transparent fluids. The other disadvantage of this flow meter is associated with the configuration of its magnetic means, consisting of a coil, wound around the core. This construction necessitates that the mass of the ball is comparable with the mass of the core and therefore significant currents might be required to ensure retaining the ball. This requirement renders the flow meter less sensitive to variation of the magnetic force and therefore to variation of the flow rate.
Besides, since the above construction of the magnetic means does not prevent the ball from reciprocating motion and therefore the measurement is less accurate.
Even more sophisticated flow meter based on similar principle is disclosed in U.S. Pat. No. 5,578,763. This flow meter comprises a conduit, through which the fluid flows, a stop in the conduit and a float body, movably received in the conduit. The float body is normally kept by the flow in engagement with the stop. At least a portion of the float is made of magnetically responsive material. An electromagnet, configured as a wire wound around the dedicated core is located outside the conduit. The poles of the electromagnet impose a magnetic force on the float body once a ramp current generator energizes the electromagnet. An auxiliary multi-turn sensor coil or a magnetic field sensor is wound around the core and is capable to detect changes in the magnetic flux field of the electromagnet, associated with movement of the float body away from the stop and towards the electromagnet. The changes of the magnetic flux are converted into sensing signal, corresponding to the current that caused attraction of the body away from the stop against the force of the fluid flow.
This flow meter does not require any more the use of a transparent conduit and it can be used to measure flow rate of non-transparent fluids.
Nevertheless, it has the same intrinsic disadvantages as the above-described flow meter, since it also employs magnetic means, which is configured as a coil wound around a dedicated core.
In conclusion it should be emphasized that despite the fact that different magnetic flow meters have been devised there is still exists a need in a new and improved flow meter, which will be suitable for accurate and reliable measurement of wide range of volume or mass flow rate.
The object of the present invention is to provide a new and improved magnetic mass and volume flow meter, which sufficiently reduces or overcomes the above-mentioned drawbacks of the known in the art magnetic flow meters, while preserving their advantages.
In particular, the main object of the present invention is to provide a new and improved mass and volume magnetic flow meter which has simple construction, is inexpensive and which is capable to measure wide range of flow rate.
The other object of the invention is to provide a new and improved magnetic flow meter, which sensitivity and accuracy is comparable with that of the known in the art flow meters and which is capable to operate with fluids, consisting of more than one phase.
The above and other objects and advantages of the present invention can be achieved in accordance with the following combination of its essential features, referring to different embodiments thereof.
According to one of the preferred embodiments, the flow meter of the invention comprises
a tubular conduit for passing a fluid therethrough,
a float member, disposed within the conduit with possibility for displacement away from a neutral position by a drag imposed thereon by the fluid, at least a portion of said float member is made of magnetically responsive material,
a differential transformer means, consisting of two separate coils connected to a variable resistor, said differential transformer means is capable to output a responsive electrical signal upon displacement of the float member from the neutral position,
a magnetic means, which is associated with the conduit and is capable to exert on the float member a magnetic force sufficient to retain the float member in the neutral position,
a source of alternating voltage connected to said differential transformer means,
a signal processing means, capable to process the responsive signal produced by the differential transformer means and to output thereof as a control current signal for controlling the magnetic means,
a source of reference voltage,
a calculating means for deriving the flow rate from the control current signal, the input of the calculating means is connected to the output of the signal processing means and to the output of the source of reference voltage,
a display means for visual representation of the derived flow rate value,
wherein said magnetic means is configured as a solenoid, which closely surrounds the conduit, the inwardly facing surface of the solenoid matches the outwardly facing surface of the conduit, said solenoid resides between the coils of the differential transformer means and is connected to the output of the signal processing means. According to the further embodiment, the signal processing means consists of connected in series a subtracting means, a first amplifier, a rectifier, a R-C circuit, a second amplifier and a means for converting the responsive signal into control current signal. As per still further preferred embodiment the coils of the differential transformer means are separated from the solenoid by insulation inserts.
As per yet another preferred embodiment the conduit is configured as a cylinder, made of non-ferromagnetic material and within the conduit are provided two opposite stopper means, limiting the displacement of the float member from the neutral position. In yet another preferred embodiment the float member is configured as elongated body and on the outwardly facing thereof are made ring-like protrusions to center the float member substantially concentrically with the longitudinal axis of the conduit. In still further preferred embodiment within the float member is removable received a disc-like insert provided with a calibrated orifice, said insert is received within the float member substantially concentrically with the longitudinal axis of the conduit.
The present invention in its various embodiments has only been summarized briefly. For better understanding of the present invention as well of its advantages, reference will now be made to the following description of its embodiments with reference to the accompanying drawings.